The present invention relates to producing wood and wood fiber products, such as plywood or hardboard, having improved resistance to fire; and more particularly to a novel method of impregnating wood articles with fire resistant ammonium phosphates.
The building industry has long recognized that the production of wood and wood fiber products such as hardboard and plywood that are rendered fire resistant broadens their field of use. Many proposals have been made to incorporate various fireproofing agents in such products. For example, materials such as borax, phosphates, and similar agents have been added both during production of wood products and by impregnation of the finished product. The prior art methods, however, are not completely satisfactory.
Addition during the production of hardboard occurs prior to the conventional pressing operation; and causes problems in pressing due to the premature decomposition of the fire retardant materials at press temperatures or to interference with the fiber-to-fiber, or lignin, bond. Further it is difficult to obtain sufficient retention of the added agents to impart any great degree of fire resistance to the product.
Methods of impregnating the finished product also raise problems. Solvent solutions of the treating agent may be used, but give rise to problems of solvent flammability and other problems involved in the use of hazardous chemicals. Post-manufacture impregnation with water soluble salts, such as the monoammonium or diammonium phosphates, pose quite a problem in maintaining the smooth pressed surface of the board, since water and aqueous solutions tend to raise and roughen wood fiber surfaces as discussed in U.S. Pat. Nos. 3,318,725 and 3,505,102. The use of aqueous solutions further requires a drying cycle to remove water absorbed into the wood fibers during treatment. This necessitates expensive equipment, and is time consuming. For example Green et al U.S. Pat. No. 2,766,139 calls for a drying or baking time in a forced air oven of 12-16 hours. If a drying cycle takes too long, the process is uneconomical. Further, if a drying cycle is too fast, then the salt tends to migrate to the surface of the board, with the consequent formation of bubbles or blisters of crystallized salt on the board's surface. Also when aqueous solutions are used, the board tends to swell in thickness.
It is also known to give other treatments to wood which do not relate to imparting fire resistance, such as Kobbe U.S. Pat. No. 1,599,135 discloses immersing wood into molten sulfur in an open tank to render the wood resistant to rot. There is no suggestion that any fire resistance may be imparted thereby; and the surface of the wood is either charred to charcoal or, to avoid charring, the wood must be immersed at a lower temperature for the first five or six hours.
Stamm U.S. Pat. No. 2,296,316 discloses treating wood to prevent shrinkage by heating the wood at temperatures between 500.degree.-600.degree.F and eliminating surface oxidation by immersing the wood in a liquid bath of molten metal; heated oil; or a fused mixture of sodium and potassium acetate. The board is then scrubbed to remove surface metal or fused salt and surface char. The process takes considerable time and there is no suggestion of any possible fire resistance imparted thereby.
Gurgiolo U.S. Pat. No. 3,501,339 mentions in column 2, soaking wood in a phosphite or phosphonate when it is a liquid; but gives no operable embodiments thereof other than as solvent or aqueous solutions.
The above processes either relate to processes of treating wood, but not specifically imparting fire resistance, or suffer various disadvantages such as incorporating insufficient quantities of fire retardant material; involving too lengthy or complex treatment steps and cycles; involving very costly and complex special equipment; or impairing the surface appearance and quality of the wood.